1. Field of the Invention
The present invention relates to a multi-layer ceramic substrate, more specifically, to a multi-layer ceramic substrate manufactured by laminate ceramic layers and metal layers one upon the other, firing the laminate, and then forming a conducting layer on the fired laminate, and a process for manufacturing such a multi-layer ceramic substrate.
2. Description of the Related Art
Various methods of manufacturing a multi-layer ceramic substrate have been developed, and the green sheet method is most often used because of its suitability when increasing the number of layers. The green sheet method is classified into two groups, a printing method and a tape method, in which the printing method is used in a condenser-including circuit substrate or the like, because the thickness of the ceramic layer is thinner than that of the tape method, and thus the parasitic capacity is larger.
In the printing method, for example, three alumina paste layers and a metal paste layer are alternately laminated by printing, the stack having three alumina paste layers on top, and the laminate is then fired. After the firing, a top conducting metal layer is formed on the top alumina layer (the three alumina paste layers become an alumina layer after firing) by a printing method (thick film forming method). The inventors found that, if the alumina paste layers printed on the top of the laminate contain contaminations such as dust, squeegee, rubber, powder, the contaminations are also fired to form defects such as pin holes in the top alumina layer. These defects may cause a short circuit between the inner metal layer in the laminate and the top conducting metal layer formed later on the top alumina layer by the printing method and thus the reliability of the insulation of this multi-layer substrate is lowered.
A similar problem occurs when the green sheet method is adopted. In the green sheet method, since the thickness of a ceramic layer is thick, typically about 100 microns, a short circuit is not easily formed by a defect formed in the ceramic layer, but there are cases wherein the thickness of the ceramic layer is thinner and large defects may be formed by contaminations in the green sheet or an extraordinary grain growth may occur, and thus a short circuit may be formed due to these defects.
The object of the present invention is to solve the above problem and increase the reliability of the insulation of a multi-layer ceramic substrate.